In wafer scanners of new generation, grating-based optical position-measuring devices are used to measure the position of the moving wafer table relative to a fixed so-called metrology frame. In that context, the scanning units of the respective position-measuring devices are located on the moving wafer table or traversing table and measure its position in all six spatial degrees of freedom. Such wafer scanners are highly dynamic machines, that is, the moving wafer table moves with traversing velocities v>1 m/s and is accelerated with a multiple of the gravitational acceleration. At the same time, the demands in terms of measuring accuracy on the optical position-measuring devices used are in the range of a few atomic diameters.
In order to realize the accuracies called for, interferentially operating position-measuring devices as described, for example, in European Published Patent Application No. 1 762 828 and U.S. Pat. No. 7,573,581, each of which is expressly incorporated herein in its entirety by reference thereto, are used on the wafer table. In such position-measuring devices, a scanning beam path is formed between a measuring standard and a scanning unit and is used to generate displacement-dependent signals. To that end, light is emitted by a scanning unit to the measuring standard, where it is split into +/−1st orders of diffraction or diffraction arms. The spatially separate orders of diffraction are reversed in direction in the scanning unit, and arrive again at the measuring standard, where they interfere. The resultant interference signal may then be evaluated with regard to the position of the objects movable relative to each other.
The position information is encoded in the phase relation of the two orders of diffraction relative to each other. The result is that, in addition to the position phase of the diffraction grating of the measuring standard, the phase lag of the two diffraction arms relative to each other also goes into the measured position value. For example, this phase lag is influenced by variations in the refractive index, which come about due to air turbulences between the measuring standard and scanning unit, whose expansion is less than the distance of the two orders of diffraction relative to each other. These turbulences in turn are caused mainly by the movement of the wafer table on which a plurality of scanning units are usually located. This position noise, or jitter, produced by air turbulences, is the cause for the greatest portion of non-correctable errors of grating-based position-measuring devices in highly dynamic machines. This being the case, the problem faced in such practical applications is to minimize or even completely eliminate this share of errors in the position measurement.
In typical, interferentially operating, optical position-measuring devices, the measuring standard is illuminated by a collimated beam of rays having a diameter of 1 to 3 mm. Typical angles of diffraction of the partial beams of rays of first order of diffraction reflected back by the measuring standard amount to 15 to 30°. The separation of the two orders of diffraction, which are used to generate the phase information of interest, thus increases with the distance to the measuring standard according to the following mathematical interrelationship:d=2*tan(φ)*h where d represents the spacing of the two partial beams of rays, φ represents the diffraction angle of one order of diffraction, and h represents the distance to the measuring standard.
The size of individual areas having the same refractive index (size of the turbulence) in the case of air moving with a flow velocity of approximately 1 m/s is typically 2 to 5 mm.
Variations in the refractive index enter significantly into the measured phase when the spatial separation of the two later-interfering partial beams of rays is greater than the size of the turbulence. Partial beams of rays which substantially overlap and whose spatial separation is markedly smaller than the size of the turbulence see the same refractive-index variation, so that no position measuring error results.
For typical systems, the measured position is thus not a function of variations in the refractive index, so long as this variation is limited to areas in the immediate vicinity of the measuring standard:d<<rT,and therefore:h<<rT/(2*tan(Φ)),typicallyh<<1.7 mm
In these equations, rT represents the size of areas having the same refractive index.
Consequently, in proximity of the measuring standard up to a distance d≈1.7 mm, the separation of the two partial beams of rays is smaller than the typical size rT of an area having homogeneous refractive index, so that no further measures are necessary to stabilize the refractive index in this area. In areas which are further away from the measuring standard, the partial beams of rays diffracted back by the measuring standard are separated so far spatially from each other that they see different air turbulences, and these air turbulences may lead to different variations in the refractive index and consequently to fluctuations in the position values measured.
To address the problem described above, it is therefore necessary to keep the refractive index homogeneous in the area between the scanning unit up to a distance of approximately 1 mm from the measuring standard. Homogenization is not absolutely necessary in the immediate vicinity of the measuring standard.
A simple manner of reducing this unwanted influence is to minimize the scanning distance between the measuring standard and the scanning unit. Thus, the air is only able to disturb in a small volume, and the accumulated phase error is slight. The aim in doing this would be to limit the free air volume to the extent that all beams of rays in the scanning beam path lie so close together that the typical size of an air turbulence exceeds this expansion. Thus, all beams of rays see approximately the same refractive index. However, the necessity of a large scanning distance in certain locally limited areas of the machine, e.g., in order to create the space necessary for a robot arm in the case of wafer exchange or the like, and thus to avoid collisions, often runs contrary to this. A large scanning distance is likewise necessary in certain operating conditions when, for example, a traversing table having greater initial tilting tolerances (since not yet regulated to the position-measuring devices) is moved under the plates having the measuring standards, e.g., when shifting the wafer table from one position in the machine to another, or else when the table must execute an emergency stop. However, all these situations take place in an operating mode of the machine which does not demand the highest accuracy or in which the table is moved with markedly lower traversing velocities, and thus the air turbulences are considerably less.
To address this problem, it is conventional to condition the air in the optical beam path by what are referred to as air showers. The attempt is thereby to generate the most homogeneous as well as constant refractive index as possible in the scanning beam path of the position-measuring device. This conditioning may be accomplished in two manners.
First of all, for instance, the air may be conditioned in the complete travel range of the wafer table, as is done in the case of interferometers, for example. When working with grating-based position-measuring devices, however, the measuring volume may also be conditioned only locally between the scanning unit and measuring standard. The aims of these air showers are to reduce slow and large or spatially broadly expanded variations in refractive index, and secondly, to avoid other air turbulences due to the movement of the traversing table, which is accomplished by as laminar a flow as possible and/or shielding of the air pushing between the scanning unit and the measuring standard due to the movement of the traversing table, by the air flow from the air shower. Comparatively great flow velocities of the air are necessary to achieve these goals. The disadvantage in such a solution is that the air cannot be perfectly homogenized. Due to the movement of the wafer table, local turbulences still arise, and thus variations in the refractive index, which produce deviations in position. In addition, due to the high flow velocities out of the air shower regardless of the traversing velocity of the wafer table, variations in the refractive index arise which result in a generally higher measuring noise.
Secondly, it is possible to reduce the variations in refractive index by employing special gases whose refractive index is less dependent on the pressure. Operation in vacuum is also possible. However, both solutions are only made possible by a very great degree of technical complexity.
The current possibilities for conditioning the air thus limit the position accuracy of the position-measuring devices used, and represent the greatest remaining portion of measuring error in determining position.